Hypothesis: Our previous studies using elastase-perfusion and angiotensin II models of abdominal aortic aneurysms (AAA) have shown that female rodents have a decreased incidence and size of aneurysm formation. We have also recently shown that human placental mesenchymal stem cells (MSCs) are protective in a mouse model of AAA. In the current proposal, we will investigate the role of dietary phytoestrogen as a preventive therapy, and its synergistic effects with MSCs as a treatment strategy to protect against AAA. Methods: We will use an elastase-perfusion and angiotensin II murine model of AAA using male and female wild-type, estrogen receptor knockout mice (ER-?--/- and ER--/-) and ApoE-/- mice. Dietary phytoestrogen will be administered to male and female mice via estrogen-rich or estrogen-free chow. Various subsets of female MSCs (placenta-, bone marrow- or adipose-derived) primed with or without estrogen will be administered intravenously or by aortic implantation in mice models of AAA. Aortic diameter will be measured on day 3, 7 and 14 (elastase perfusion model) and day 28 (angiotensin II model). Aortic tissue will be harvested to analyze pro-inflammatory cytokine (IL-17, TNF-?-, MCP-1, IL-1--, KC and RANTES) and HMGB1 (high mobility group box 1; a pro-inflammatory nuclear protein) production by ELISA, MMP2 and MMP9 activity by zymography, serine proteases (uPA, tPA and PAI-1) by western blots, paracrine factors (VEGF, HGF, PGE2) by ELISA, elastin and collagen degradation as well as aortic smooth muscle expression by histology, and immune cell (macrophages, CD4+ T cells, neutrophils) infiltration by flow cytometry. Results: Preliminary results demonstrate a significant upregulation of estrogen receptor (ER-?) expression in female mice on days 1, 3 and 14 after elastase-perfusion compared to males, as well as in aortic tissue from human females compared to males after AAA. Also, male mice fed a diet rich in estrogen display a significantly decreased aortic diameter, decreased cytokine production (IL-23, IL-1--, IL-6 and IL-27), decreased MMP2 and 9 expression and decreased macrophage and neutrophil infiltration compared to male mice fed an estrogen free diet. Furthermore, treatment with human female MSCs attenuates aortic diameter, pro-inflammatory cytokine production and cell infiltration after AAA. Female MSCs inhibit HMGB1 and IL-17 production more significantly than male MSCs. Also, estradiol-priming of female MSCs offer significantly increased protection from vascular inflammation in aortic tissue from male AAA patients. Conclusions: Dietary estrogen therapy and mesenchymal stem cells can attenuate aneurysm formation and inflammation in the elastase-perfusion murine model of AAA and in human aortic tissue from AAA patients. We propose to delineate the phytoestrogen mediated anti-inflammatory effects, and its crosstalk with various subsets of gender-specific MSCs, on aortic aneurysm formation in the murine (elastase-perfusion and angiotensin II) models as well aortic tissue and cells from AAA patients.